This invention relates to the field of image sensors, and specifically to low noise operation of active pixel arrays.
Active pixel sensor (APS) arrays are solid state image sensing structures, typically manufactured using CMOS technology. They are distinguished by integrating some form of active electronic circuitry within each pixel to produce a buffered or amplified representation of small signal charge induced by photoelectric conversion in the pixel. Typically the pixel output takes the form of a low-impedance voltage signal.
CMOS image sensors are used increasingly for consumer imaging applications, including film replacement in still cameras. For this application a high dynamic range is desirable, and this implies low noise operation. Typically, whole camera performance is desired to have pixel-referred noise lower than 100 rms ((root-mean-square) electrons, with peak signal levels of the order of 100,000 electrons.
The "active pixel" CMOS architecture is well suited to this application because it has low read noise and in general has imaging properties similar to traditional Charge-Coupled Devices (CCDs). Moreover, CMOS fabrication technology is more widely available than CCD, and has other attractive characteristics, such as lower power consumption and greater fault tolerance.
Active pixel CMOS architectures have been disclosed over a considerable period. A known disadvantage of this technology is its suceptibility to fixed pattern noise due to random voltage offsets in some transistors used in the pixels. Where such fixed pattern noise is present, the resulting images produced by the camera are marked by a fixed speckled pattern.
U.S. Pat. No. 4,942,474 (Akimoto) and U.S. Pat. No. 5,471,515 (Fossum) teach how to achieve a high degree of cancellation of the fixed offsets which would otherwise limit performance. Both of their techniques use column-based double correlated sampling to remove fixed offsets in pixel and column buffers. This involves the use of at least two capacitors in each pixel column of the array. Moreover in the image sensors described in U.S. Pat. No. 4,942,474 reset noise is present in the image sensor output signals. Pixel reset noise occurs as a consequence of resetting operations carried out on pixels in the array and is a function of temperature and photodiode capacitance in the pixel.
U.S. Pat. No. 5,471,515 further proposes the use of a photogate structure within each pixel to also enable cancellation of pixel reset noise. However, this scheme suffers from the disadvantage that such photogate pixels are larger in size than simpler photodiode pixels. Also, the image sensing area is covered with polysilicon which attenuates the sensitivity of the pixels, especially in the blue region of the visible spectrum. Moreover, dark current fixed pattern noise is still present in the resultant image produced by the camera from the individual pixels. Dark current fixed pattern noise is the term commonly used to describe electronic signal noise in the pixel outputs which is independent of the image being sensed and which is present even when the pixel array is not being exposed to an image i.e. in the "dark" condition where substantially no light is incident upon the image sensing area.